In general, dial gauges are used to make comparisons and measurements within a very small range. To avoid misreading the graduation, there has already been proposed a so-called one turn type dial gauge wherein a needle is limited to less than one turn in its range of rotation. Such a one turn type dial gauge can avoid misreading the graduation and be convenient to make comparisons and measurements, in that the needle is adapted to turn through a predetermined angle within one turn, normal and reverse graduations are provided which divide the range of rotational angle of the needle into two, and an ungraduated portion is provided between the normal and reverse graduations.
However, with the one turn type dial gauge as described above, when the dial plate is mounted to an inner frame in a manner to be unlimitedly rotatably adjustable, the needle may reach the normal or the reverse graduation going beyond the ungraduated portion depending upon a zero adjustment position at the time of setting the dial gauge. Therefore, to obviate such disadvantage as described above, the present applicants have proposed such a dial gauge that the dial plate and the needle are determined in their range of turning in such a manner that the needle may not go beyond the ungraduated portion of the dial plate (Refer to Japanese Utility Model "Kokai" (LaidOpen) No. 6894/1980). This invention is one in which the proposed dial gauge has been further developed. More specifically, in the proposed dial gauge, the dial plate is unrotatably secured to an outer frame, and the outer frame secured thereto with the dial plate is controlled in turning with respect to the inner frame, whereby the dial plate is controlled in turning. Hence, such disadvantages have been presented by the proposed dial gauge that it is necessary to very strictly control and adjust a positional relationship in mounting between the outer frame and the dial plate, and further, positional relationship in mounting between parts other than the outer frame and the dial plate at the time of assembling, and moreover, once the dial unit is assembled, it becomes difficult to make fine adjustment and disassembling. Furthermore, the construction as a whole has been complicated.
To state more detailedly, the dial gauge must be assembled with all of the parts being registered with one another in such a manner that the dial plate must be solidly secured to the outer frame in a predetermined positional relationship therebetween, while, the needle must be held in a predetermined positional relationship with a spindle and an enlarging-transmitting mechanism for enlarging and transmitting a displacement of the spindle. For example, if predetermined marks are not put on the dial plate and the outer frame in relation to the inner frame, then an improper positional relationship between the needle and the range of turning control occurs from one dial gauge to another, and, in some cases, the dial gauge may be disadvantageously assembled with the needle deviating from the range of turning control. Moreover, once the dial gauge is assembled, it has been very difficult to make fine adjustments and disassembling.